The present invention relates to a circuit configuration for an anti-lock brake system equipped with sensors for determining the rotational behavior of the driven wheels and comprising circuits for deriving from the sensor signals a vehicle reference speed which serves as a reference value for controlling the braking pressure in dependence on the rotational behavior of the individual wheels.
A great variety of anti-lock brake systems of this type are known. The information about the rotational behavior of the wheels and about the vehicle behavior required for the anti-lock control or braking pressure control is obtained by measuring the wheel rotational behavior using wheel sensors and by logically combining the sensor signals. Acceleration sensors, pressure sensors, and the like are also used in some cases. A vehicle reference speed, which serves as a reference value for judging the rotational behavior of the individual wheels and, hence, for controlling the braking pressure, is obtained by selecting and logically combining the wheel sensor signals, by interpolating or extrapolating the signals if no expressive information is available for a short time, and by limiting the speed gradient to the physically possible values or to other more expedient values of the vehicle deceleration and vehicle acceleration.
In order to reduce the manufacturing expenditure, great effort is made to use the smallest possible number of sensors, although the control quality depends on the assessable information. When only relatively few data are available for the control because few sensors are used, it is often difficult to correctly interpret the measured signals. One difficulty in assessing the wheel rotational behavior measured by the sensors results because the wheels are coupled with each other and with the drive motor. Due to its drag torque, the drive motor affects the wheel rotational behavior and, hence, can induce wrong information or misinterpretations.
It is an object of the present invention to diminish the influence which the engine drag torque has on the control in an anti-lock system equipped with only few sensors. This problem is especially significant if only the two driven wheels of a vehicle are furnished with wheel sensors or, in the case of all-wheel drive, where all wheels are interconnected by the drive shaft and, hence, act synchronously to a large extent.